The present invention is a method and apparatus for removing undesired particles, such as fly ash, from gas streams. More particularly, the present invention embodies an improved approach for removing such undesired particles by selectively introducing oxidants into the gas stream.
Environmental standards for particulate emissions from coal-fired electrical power plants, petroleum refineries, chemical plants, pulp and paper plants, cement plants, and other particulate-emitting facilities are becoming increasingly more demanding. For example, air quality standards in the United States now require power plants to remove more than 99% of the particulates produced by coal combustion before the flue gas can be discharged into the atmosphere. The term xe2x80x9cparticulatexe2x80x9d within the meaning of these restrictions generally refers to fly ash and other fine particles found in flue gas streams and can include a host of hazardous substances such as those listed in 40 CFR xc2xa7302.4 (e.g., arsenic, ammonia, ammonium sulfite, heavy metals and the like. As environmental standards tighten, there is a corresponding need for a more efficient means of particulate removal.
An electrostatic precipitator is a commonly used device for removing electrically particulates from the gas streams produced by plants and refineries. In a typical electrostatic precipitator, undesired particle-laden gases pass negatively charged corona electrodes which impart a negative charge to the particulates. The charged particulates then migrate towards and collect on positively charged collection plates and are intermittently removed by various techniques, including sonic horn blasts or rapping of the collection plates. Electrostatic precipitators may employ a common stage or separate stages for both the charging and collection of particulates.
In utility applications, there are two types of electrostatic precipitators. Cold-side electrostatic precipitators are located on the downstream side of the air preheater or heat exchanger (which transfers heat from the flue gas to the air to be fed into the furnace) and therefore operate at relatively low temperatures (i.e., temperatures of less than about 200xc2x0 C.). Hot-side electrostatic precipitators are located on the upstream side of the air preheater and therefore operate at relatively high temperatures (i.e., at least about 250xc2x0 C.).
Many hot-side electrostatic precipitators suffer from problems related to the resistivity of collected particulates. Such problems can cause a deterioration of the particulate collection efficiency of the electrostatic precipitator (and higher particulate emissions) and excessive ESP power consumption. These problems can be caused by sodium depletion of collected particulates on the collection plates. It has long been known that ionic charge transfer through the collected particulate layer at high temperatures can be degraded as available charge carriers, namely sodium cations, are depleted from the collected particulate layer. This phenomenon is commonly referred as sodium depletion. The problems can also be caused by the inherently high resistivity of particulates and/or resistivity problems during low load or at colder temperatures.
Additives, such as sulfur trioxide, ammonia, and various surface conditioning additives (such as sulfuric acid) that are effective under cold-side conditions are generally ineffective under hot-side conditions because of different charge conduction mechanisms. Referring to FIG. 1, under cold-side conditions (which exist at operating temperatures less than the critical temperature) surface conduction of charge is believed to be the predominant charge conduction mechanism while under hot-side conditions (which exist typically at operating temperatures more than the critical temperature) volume conduction of charge through the particulates is believed to be the predominant charge conduction mechanism. As used herein, the xe2x80x9ccritical temperaturexe2x80x9d is the temperature corresponding to the highest attainable resistivity of a particulate (which is commonly located at the top of a bell-shaped curve as shown in FIG. 1). As can be seen from FIG. 1, particulate electrical resistivity varies with temperature over as much as two orders of magnitude at normal process temperatures for hot-side ESPs.
For the reasons set forth above, an effective flue gas conditioning treatment under hot-side conditions therefore should prevent or substantially delay long-term ion (e.g., sodium) depletion and moderate resistivity for highly variable particulate compositions and process temperatures.
One conditioning method for controlling particulate resistivity that has had some success under hot-side conditions has been bulk addition of sodium into the coal feed to the boiler. Typically, from about 0.5 to about 4% by weight sodium (relative to the weight of the ash in the coal) is added to the coal feed as a sodium sulfate or soda ash. The sodium is co-fired with the coal in the boiler resulting in the sodium being incorporated into the particulates as sodium oxides.
The bulk addition of sodium to the coal feed can, however, cause problems. For example, bulk sodium addition can cause boiler slagging and boiler and economizer fouling due to the high sodium content of the particulates (substantially negating any gains by reduced ESP cleaning). Bulk sodium addition can lead to the consumption of excessive amounts of alkali material (and a commensurate increase in operating costs) and to higher gas temperatures downstream of the boiler (that can lead to duct and electrostatic precipitator structural problems). Bulk sodium addition may not effectively control sodium depletion, because the added sodium charge carriers are contained as sodium oxides in the bulk particles. Therefore, a thin layer of collected particulates next to the plate that is never rapped clear (or removed from the plate) will still be subject to sodium depletion and higher electrical resistivity as the sodium ions in the collected thin layer migrate to the plate. Once sodium depletion is established in the collected thin particulate layer, bulk sodium addition becomes less and less effective over time. Compared to the absence of bulk sodium addition, it is common that the prolonged bulk addition of sodium may delay or extend time between ESP cleaning by only a few months. Bulk sodium addition often cannot be performed on an intermittent or as-needed basis and thereby fails to provide control over short-term particulate resistivity. The sodium content of the coal supply is a major contributor to the electrical resistivity of the resulting fly ash. The sodium content can range from less than 0.5% to more than 2% depending on the coal supply. Coal sodium content is variable over a period of days to weeks with a lag time of several hours from when new coal is loaded into the feed bunkers to its full effect on ESP performance. There is no real-time feedback to determine optimum sodium content in the as-fired coal. Therefore, the bulk sodium addition rate is adjusted based on observed changes in stack opacity and ESP power. Bulk sodium rate adjustments are made several hours after as-fired coal changes and the effects of a rate change do not take effect for several additional hours.
Another hot-side conditioning method is to inject sodium-precursor chemicals, notably carboxylic acid salts, into the flue gas stream as a finely atomized liquid spray. This conditioning method is discussed in detail in U.S. Pat. No. 6,267,802. The conditioning mechanism is enrichment of sodium ion charge carriers on the collected particulates. Advantages compared to bulk sodium addition to the coal include the co-precipitation of chemical and sodium ion charge-carriers with the particulates, the use of only a small fraction of the material required for bulk sodium addition, the avoidance of detrimental boiler slagging and fouling, and the rapid and precise adjustment of additive application rate.
Sodium precursor chemicals, however, may be unable to address short-term ESP performance problems related to load, coal, resistivity and gas temperature. Sodium precursor chemicals sometimes cannot overcome severe short-term resistivity changes associated with temperature swings during unit load changes and can be less effective on the lower temperature hot-side ESPs because the inherent particulate resistivity is higher.
Yet another hot-side conditioning method that has had some success under hot-side conditions has been the introduction of sodium nitrate into the flue gas stream. Sodium nitrate conditioning additives have been sold under the trade name xe2x80x9cCOMBUSTROL FACT 5000xe2x80x9d by Calgon Corporation and are discussed in U.S. Pat. No. 6,001,152. The sodium nitrate is dissolved in a liquid, and the liquid additive is atomized and introduced into the flue gas stream.
A problem with the use of sodium nitrate in the flue gas stream is the lack of long-term control over ash resistivity. Although the sodium nitrate additive can produce a significant, initial decrease in resistivity, it has been observed that the decrease in resistivity rapidly degrades over time and returns to unconditioned particulate resistivity levels. Accordingly, a relatively high amount of the additive is required to realize acceptable levels of ESP performance, leading to higher operating costs when compared to sodium precursor chemicals. Sodium nitrate is also ineffective when process temperatures are above about 725xc2x0 F. (385xc2x0 C.) due to rapid thermal decomposition.
These and other needs are addressed by the additive(s) of the present invention. Generally, the additives of the present invention utilize metal nitrate(s) and/or nitrite(s) to provide effective conditioning of particulates under both cold-side and hot-side conditions.
In one embodiment of the present invention a process is provided for removing undesired solid particles (e.g., particulates) from a gas stream (e.g., a flue gas stream) that can realize these and other objectives. The process includes the steps of:
(a) contacting (e.g., injecting) with the gas stream a composition including a solid or liquid additive composition that preferably includes potassium nitrate and/or nitrite and optionally one or more other metal (other than potassium) nitrates and/or nitrites;
(b) collecting on at least one collection surface in a collection zone a solid agglomerate including at least a portion of the additive composition or a derivative(s) thereof and at least a portion of the undesired solid particles; and
(c) removing the agglomerate from the collection surface. As used herein, xe2x80x9cagglomeratexe2x80x9d refers to a cluster or accumulation of undesired particles and additive particles and xe2x80x9ccondensation temperaturexe2x80x9d refers to the temperature at which a given vapor component of a gas stream condenses into a liquid under ambient pressure.
The additive is particularly effective under hot-side conditions. The temperature of the gas stream under hot-side conditions is typically at least about 250xc2x0 C. (480xc2x0 F.), more typically ranges from about 270xc2x0 C. (520xc2x0 F.) to about 480xc2x0 C. (900xc2x0 F.), and even more typically from about 177xc2x0 C. (350xc2x0 F.) to about 400xc2x0 C. (750xc2x0 F.)
Surprisingly and unexpectedly, potassium nitrate has proven more effective than sodium nitrate in lowering collected particle resistivity over both short- and long-term periods. For example, the additive, due to its higher degree of thermal stability, can provide long-term resistivity enhancement at temperatures of more than 725xc2x0 F. (385xc2x0 C.) and up to about 800xc2x0 F. As noted, a prominent theory for the occurrence of high resistivity in electrostatic precipitators is the sodium depletion theory which holds that high resistivity develops in the accumulated undesired particle layer because of the migration of sodium ions towards the collection plates, thereby increasing the resistivity of the accumulated particle layer. Surprisingly and unexpectedly, when potassium is compounded with the nitrate anion, the metal cations and nitrate anions migrate freely throughout the layer and provide significant, long term reductions in the resistivity of the collected, undesired particle layer.
The additive in the composition can be nontoxic and substantially odorless. An additive is typically deemed xe2x80x9cnontoxicxe2x80x9d if the presence of the additive in the resultant agglomerate does not cause the agglomerate to be environmentally unacceptable under the standards and procedures set forth in the Toxicity Characteristic Leaching Procedure (xe2x80x9cTCLPxe2x80x9d) established by the United States Environmental Protection Agency. The TCLP provides analysis procedures for waste materials to detect environmentally unacceptable levels of substances, including inorganic elements, volatile organic compounds, and semi-volatile organic compounds. The TCLP specifies the maximum acceptable concentration for such substances. An additive is deemed to be xe2x80x9codorlessxe2x80x9d if the presence of the additive in the agglomerate cannot be detected by the human nose.
Because of the solubility limits of the potassium nitrate and nitrites in the selected solvent (e.g., water), other types of metal nitrates and/or nitrites can be incorporated into the additive to provide a higher effective concentration of the nitrate and/or nitrate anion. In one formulation, the additive includes potassium nitrate, potassium nitrite, and nitrates and/or nitrites compounded with other metals. In another formulation, the additive includes potassium nitrate and one or more of sodium, calcium, and aluminum nitrate. In yet another formulation, the additive includes potassium nitrite and a metal selected from Groups 1, 2, 6, 7, 8, 9, 10, 11, 12 and 13 of the Periodic Table and preferably one or more of sodium, calcium, and aluminum nitrite. In yet a further formulation, the additive includes only potassium nitrate and/or nitrite and no other metal nitrates and/or nitrites. In other formulations, the additive includes not only the salt but also the mineral acid precursor of the salt.
The liquid additive, as introduced into the gas stream, preferably includes at least about 0.5 wt. % potassium nitrate and/or nitrite and more preferably from about 1 to about 6 wt. % potassium nitrate and/or nitrite. The liquid additive can further include at least about 0.5 wt. % of other metal nitrates and/or nitrites and more preferably from about 2 to about 8 wt. % of the other metal nitrates and/or nitrites. The molar ratio between the potassium salts and the non-potassium salts typically ranges from about 0.2:1 to about 2:1 and even more typically from about 0.5:1 to about 0.9:1.
Unlike a liquid additive, the solid additive does not suffer from the limitations of solubility and include much higher levels of potassium nitrates and/or potassium nitrites. Preferably, the metal in at least most of the moles of metal nitrates and/or metal nitrites in the solid additive is potassium.
To provide a higher level of solubility of the various nitrate (and other oxidizing) salts in the liquid additive, the liquid additive can include one or more solubilizing agent(s). A solubilizing agent is an element or compound that causes the selected salt to have a higher solubility in the solvent than is possible under the same conditions of temperature and pH, in the absence of the agent. A preferred solubilizing agent for potassium nitrate is a peroxygen compound, such as a peroxide, with hydrogen peroxide being preferred. The solubilizing agent can be used to increase solubility levels not only for nitrates and nitrites but also for any other salt that is introduced into the gas stream. Examples of such other salts include phosphates, phosphites, carbonates, sulfates, sulfites, and mixtures thereof.
In hot side applications, the additive can substantially eliminate the potential for air preheater problems (e.g., such as build up of unwanted additive/particle deposits on the air preheater), particularly when the additive is injected as a solid. When a liquid additive is sprayed into a gas stream, a deposit of undesired particles and additive can form. Such deposits commonly form at the point of injection and on metal surfaces downstream from the injection point, such as air preheaters and electrostatic precipitator electrodes. The additive, when injected into a heated, moist gas stream as a fine mist or powder, commonly produces markedly cleaner, brighter metal surfaces near the injection point than other liquid additives and such surfaces generally do not build up undesired particles. Additionally, the use primarily of salts in the additive of the present invention, may inhibit corrosion of ductwork and electrostatic collection surfaces.
The additive can be mixed with a volatile carrier fluid, such as water, which vaporizes readily at the gas stream temperature (i.e., has a boiling point that is less than the gas stream temperature) to form particles (solid and/or liquid particles) of the additive(s). It is preferred that substantially all of the carrier fluid vaporize before the salt or derivative(s) thereof contacts the collection surface, which is commonly within no more than about 2 seconds after contact of the composition with the gas stream. The concentration in the carrier fluid of the additive(s) before injection into the gas stream, typically ranges from about 0.1 to about 5 wt. %.
While not wishing to be bound by any theory, dispersed particles of the additive are believed to be discrete from the undesired particles in the gas stream. Upon contact with the collection surface, the additive particles and the undesired particles form the agglomerate. After collection, the additive is believed to do most of the conditioning of the undesired particles. The preferred residence time of the droplets in the gas stream before contacting the collection surface preferably ranges from about 0.25 to about 2.00 seconds.
Preferably, to yield a substantially xe2x80x9cdry system,xe2x80x9d the temperature of the collection surface in the collecting step is greater than both the condensation temperature of the water vapor in the gas stream and any vaporized carrier fluid. As used herein, a xe2x80x9cdry systemxe2x80x9d refers to a system that employs a substantially dry collection surface (i.e., having substantially no liquid in contact therewith) for undesired particles. The dry system can include significant amounts of water vapor.
After a predetermined degree of build-up, the agglomerate of undesired particles and additive particles may be removed from the collection surface, collected in a hopper and removed from the unit. Removal maybe accomplished by vibration of the collection surface, removing the collection surface from the collection zone, or contacting the collection surface with a reverse gas stream having a direction of flow substantially opposite to the gas stream.
In a related embodiment of the invention, an apparatus for undesired particle removal is disclosed that includes (i) a housing; (ii) an inlet and outlet for the gas stream; (iii) an injection apparatus to inject an additive composition into the gas stream; and (iv) one or more collection surfaces supportably positioned within the housing to collect both the undesired particles to be removed and additive particles which, in turn, form an agglomerate on the collection surface. The apparatus may include a plurality of collection surfaces and one or more hoppers to collect the agglomerate that is removed from the collection surface.
The additive injection apparatus is preferably a plurality of dispersion devices (e.g., nozzles) positioned within and/or across the gas stream to uniformly disperse the additive composition into the gas stream. The additive injection apparatus may be advantageously located upstream of the collection surface at a distance sufficient for a substantial portion of any carrier fluid, preferably about 90% or more by weight, to separate by vaporization from the additive particles before the particles contact the collection surface.
In an electrostatic precipitator embodiment of the present invention, the apparatus may include a power supply; at least one electrode connected to the negative terminal of the power supply and positioned relative to the input gas stream to impart a charge to the undesired particles to be removed and the additive particles; and at least one collection surface connected to the positive terminal of the power supply and positioned parallel to the flow of the gas stream.
The additive of the present invention can have a number of advantages relative to existing additives, particularly under hot-side conditions. When the additive is added to the gas stream, the electrostatic precipitator, even under hot-side conditions, can remove sufficient undesired particles to form a gas stream that is in compliance with pertinent environmental regulations. The additive can be readily employed with existing electrostatic precipitators simply and inexpensively by retrofitting the precipitator with devices, such as nozzles or drip emitters, for injecting the additive into the gas stream. The injection of the additive into the gas stream upstream of the electrostatic precipitator rather than the addition of the additive to the coal feed can be done on an intermittent or as-needed basis, avoid or substantially inhibit boiler slagging and boiler and economizer fouling, increase the efficiency of the electrostatic precipitator, reduce undesired particle reentrainment during accumulation and/or removal of undesired particles from a collection surface, require only low consumption of the additive, overcome severe short-term resistivity changes associated with temperature swings during unit load changes, effectively condition particulate resistivity not only under hot-side but also cold-side conditions, provide a fast resistivity response when compared to bulk sodium addition and sodium precursors, have no detrimental effect on the performance of concrete made from conditioned undesired particles, and generally not increase the gas stream temperature downstream of the boiler, all preferably without significantly increasing capital and operating costs.